1. Field of the Invention
This invention relates, in general, to a novel resistance welder and a novel control device for the resistance welder.
2. Description of the Prior Art
A resistance welder is a well known device wherein two or more sheets of metal to be welded are put between electrodes which apply pressure to the sheets. A current of several thousand through ten thousand or more amperes is caused to flow in the metal sheets through the electrodes in order to melt and weld the metal sheets by Joule heating due to the current flowing in the metal sheets. A control device in the resistance welder controls the effective value of the current, the weld time, and the duration of the time for applying pressure to the sheets.
FIG. 1 is a circuit diagram of a conventional resistance welder. In FIG. 1 numeral 1 designates an AC power source. The resistance welder 50 includes a pair of anti-parallel connected electronic contactors, such as the thyristors 2a and 2b, a welding transformer 3, and secondary circuits 5 and 5A of the welding transformer 3. The resistance welder 50 also includes a control device 60 which includes a current transformer 4, an analog to digital converter 6 (hereinafter referred to as an A/D converter), a microprocessor 7, and a program circuit 8.
The AC power source 1 supplies an AC power of, for example, 400 V or 440 V, 50 Hz or 60 Hz, to the welding transformer 3 through the thyristors 2a and 2b. When a current flowing through the primary winding of the welding transformer 3 is I amperes and the winding ratio of the transformer is n:1, the total current flowing through the secondary circuits 5 and 5A is n.times.I amperes. The program circuit 8 is a device for setting reference effective values of the current flowing through the secondary circuits 5 and 5A, respectively, and for setting the weld time. The set values in the program circuit 8 are applied to the microprocessor 7 and are stored in a main memory (not shown) of the microprocessor 7. The microprocessor 7 calculates the primary current I of the welding transformer 3 based on the data stored in the main memory and determines a firing angle for the thyristors 2a and 2b.
Hereinafter the operation of the control device 60 of the resistance welder 50 will be described in more detail. FIG. 2 shows a flowing condition of the secondary current. It is shown in FIG. 2 that a current of an effective value I.sub.1 flows through the secondary circuits 5 and 5A during the T.sub.1 cycle, that no current flows during the T.sub.2 cycle which is provided for cooling, and that a current of an effective value I.sub.2 flows through the secondary circuits 5 and 5A during the T.sub.3 cycle, respectively. The program circuit 8 can set the effective values I.sub.1 and I.sub.2 of the current and the length of the cycle periods T.sub.1, T.sub.2 and T.sub.3. The primary current I flowing through the welding transformer 3 is detected by the current transformer 4 and is converted into a digital value in the A/D converter 6 thereby being fed back to the microprocessor 7. The microprocessor 7 compares digitally the current thus fed back with the reference value previously stored in the main memory, and controls the firing angle of the thyristors 2a and 2b to make the fed back current equal to the reference value based on the comparison result at every cycle of the AC power source 1. During the cooling time T.sub.2 shown in FIG. 2 or when the thyristors 2a and 2b are in the off state, no firing pulse signal is applied to the thyristors 2a and 2b by the microprocessor 7. This control technique is well known to those skilled in the art, therefore a detailed description thereof is neglected.
Since the fed back current I is an instantaneous value and the reference value is given in terms of an effective value, the microprocessor 7 is required to convert the instantaneous value of the fed back current I into an effective value. The microprocessor 7 reads the fed back current I at a predetermined sampling rate (usually between 100 .mu.S and 200 .mu.S) and calculates the effective value as follows: ##EQU1## where, Ieff: an effective value of the fed back current I,
Ik: an instantaneous value of the fed back current I at each sampling time, PA1 n: the number of sampling during a half cycle time of the AC power source 1.
As described above, the conventional resistance welder 50 employs a system for feeding back the current I flowing through the primary winding of the welding transformer 3 to control the current I such that it is constant. In the case of the resistance welder 50 shown in FIG. 1, there are provided two secondary circuits 5 and 5A. Therefore when one of them, such as the secondary circuit 5, is disconnected, a current whose value is two times that of the usual value flows through the secondary circuit 5A.
If the current of two times the standard value flows through the welding portion of the secondary circuit, sometimes problems such as the burning of a hole or the like in the welding portion, may occur. In this case the manufacture to be welded becomes inferior and must be excluded from the manufacturing process. The welding portion corresponding to the disconnected secondary circuit is of course not welded, but can be rewelded at a later time and thus can be recovered.
For example, in an automobile manufacturing industry, inferior goods produced by excessive welding occur in a process for welding a car body and the inferior goods must be removed from the assembly process thereby adversely affecting the following assembly process. Therefore even in the case where one of the secondary circuits is disconnected, it is necessary that normal welding can be executed at the other secondary circuit.